oa Heterojunction Solar Cells Exceeding 20% Efficiency Using Gettered Monolike Silicon Wafers

Silicon heterojunction (SHJ) solar modules are an attractive and suitable option for application in high temperature environment such as in the state of Qatar. This is mainly related to their high open-circuit voltage (Voc) [1], which is essential for achieving better performance at high temperatures. High efficiency SHJ solar cells are usually made from high-quality n-type monocrystalline silicon wafers, which increases the cost of the device. If lower-cost materials such as high performance multicrystalline and mono-like silicon wafers could be used, significant reduction in the cost of PV electricity can be achieved. In this work, electrical properties of industrial-scale n-type and p-type mono-like silicon wafers have been investigated for use in high efficiency SHJ solar cells. The wafer's electrical resistivity were measured by four-point probe as 1–3 Ω-cm for n-type and ∼1 Ω-cm for p-type wafers. Since minority carrier lifetimes in as-grown (un-gettered) cast silicon, including mono-like silicon is usually low (i.e. inadequate for application in high efficiency solar cells), phosphorus diffusion gettering in a POCl3 furnace and/or bulk passivation by SiNx:H were used for enhancement of electrical properties. SHJ solar cells were then fabricated using the gettered wafers and as-grown sister wafers to assess the effectiveness of the gettering treatments. Figure 1. Injection dependent minority carrier lifetime for as-grown wafers and for gettered sister-wafers, after passivation with i/n and i/p amorphous silicon layers Figures. 1 shows minority carrier lifetimes measured by Quasi Stead-State Photoconductivity (QSSPC), on wafers as-grown, gettered and gettered with SiNx:H bulk passivation. As-grown lifetimes are low, being < 170 μs for p-type wafers and < 370 μs for n-type wafers, at 1 × 1015 cm-3 injection level. After passivation, significant improvement (up to 520 μs for p-type and 1900 μs for n-type) was achieved at 1 × 1015 cm-3 injection level. Despite the significant enhancement in overall lifetime, lifetimes at low-injection level is still limited, particularly for the p-type silicon. Nevertheless, very high carrier lifetime (up to 2400 μs) was recorded at 1 × 1015 cm-3 injection level (Figure 1(a)), and a SHJ cell with VOC of 720 mV and a photo-conversion efficiency of 20.3 % were successfully achieved, by gettering and bulk passivation. In conclusion, we demonstrated that electrical properties of industrial-scale mono-like silicon wafers can be efficiently optimized by gettering and bulk passivation treatments for application in high efficiency solar cells. A silicon heterojunction solar cell with efficiency exceeding 20% was produced. References [1] S. De Wolf, A. Descoeudres, Z. C. Holman, and C. Ballif, «High-efficiency silicon heterojunction solar cells: A review,» Green, vol. 2, pp. 7–24, 2012.